The invention claimed herein relates generally to hydrostatically driven agricultural vehicles, and more specifically to a braking system for such vehicles that interconnects the normal brakes with the hydrostatic pump to more effectively slow or stop the vehicle.
Hydrostatically driven tractors, such as that generally shown in U.S. Pat. No. 6,230,830, have grown in popularity over the last several years, and can be found in use throughout the world. Machines of this type are quite reliable and easy to operate; however, there are several scenarios in which they present operational difficulties or problems that have not heretofore been fully addressed.
For example, while traveling at road speeds and pulling a trailer (such as, for example, a trailer or a beet wagon) without trailer brakes, the tractor must, according to industry standards and general safety requirements, be able to stop within a certain distance as a result of a single action or activity of the operator. On a hydrostatically driven tractor the vehicle can be stopped by either using the forward/neutral/reverse lever (hydrostats) or the service brake. To achieve acceptable roading speeds with a hydrostatically driven tractor the variable displacement hydrostatic motor must have a small minimum displacement that, unfortunately, does not provide enough dynamic braking to stop the tractor when the forward/neutral/reverse lever is returned to neutral. Actuating the service brake only, while the tractor is roading, provides insufficient dynamic braking, as the service brakes have to overcome the inertia of the tractor/trailer as well as stall the hydrostatic drive.
Another scenario arises while the tractor is traveling at roading speeds, pulling a trailer equipped with trailer brakes. The tractor must be able to be stopped within a certain distance by the operator using a single input and with the trailer brakes actuating before the hydrostats destroke (move to neutral). Returning the hydrostatic forward/neutral/reverse lever to neutral will not apply the trailer brakes, which are coupled hydraulically to the service brakes, and may cause the vehicle and trailer to jackknife. Actuating the service brakes will engage the trailer brakes, however, the stopping distance will be unacceptable as the brakes, as mentioned above, must overcome the torque of the hydrostatic transmission.
The above-described scenarios occur under circumstances that provide additional difficulties for the operator. While in a draft application, the tractor must be decelerated while the operator is actuating hydraulic levers and steering at headlands. Actuating the service brakes alone will not overcome the torque of the hydrostatic transmission and the operator will have to move the forward/neutral/reverse lever, which cannot be done concurrently with the other operations—at least not conveniently or easily. The instant invention allows the operator to actuate the service brakes to concurrently destroke the hydrostatic pump and slow the vehicle. When the service brakes are disengaged the tractor accelerates to its original speed.
It would be desirable and beneficial to provide a hydrostatically driven tractor that would overcome the above-noted disadvantages of known hydrostatic tractors.